1. Field of the Invention
The present invention relates to power supplies and more particularly to power supplies for energizing gas-filled lamps.
2. Description of Related Art
Gas-filled lamps, also referred to as gas-discharge lamps, have long been used in spectrophotometers for measuring the spectral transmission characteristics and spectral absorption coefficients, etc. of materials. By analyzing the spectral properties, one could detect, characterize, identify as well as determine the concentration of the material. Deuterium lamps which have a high output in a stable, continuous spectrum in the ultraviolet region are widely used in spectrophotometers. In general, the spectral frequency or wavelength of the ultraviolet output of the deuterium lamp is dependent upon the current but not the power supplied to the lamp.
DC (direct current) operation of deuterium lamps is most common. FIG. 1 shows a basic deuterium lamp DC power circuit used in the past for energizing deuterium lamps. The deuterium lamp 10 is schematically illustrated as having two electrodes 12 and 14 enclosed by a deuterium gas-filled bulb. One of the electrodes is an anode 12 and the other a cathode 14. The cathode 14 is in the form of a filament. The basic operation of the deuterium lamp 10 requires a DC voltage source 15 of a sufficient voltage level to be momentarily applied across the electrodes 12 and 14 to trigger ionization of the deuterium gas in the space between the electrodes thereby initializing gas discharge. Thereafter, current flows relatively freely through the ionized gas between the electrodes 12 and 14, with electrons flowing from the cathode 14 to the anode 12 under the influence of the applied voltage potential, which is substantially less than the triggering voltage as will be explained in greater detail below. The interaction of the electrons and the gas ions causes further discharge thereby sustaining the operation of the deuterium lamp.
A small power supply 16 is applied to the ends of the cathode filament 14. The cathode filament 14 serves two functions. It heats the cathode and thereby stimulates thermionic emission of electrons and subsequent gas discharge. It is also used to initially warm up the cathode prior to the initial gas discharge in order to protect the cathode from ion bombardment damage. As is known to those familiar with deuterium lamps, cathode temperature is a critical factor in determining lamp performance.
FIG. 2 shows the typical voltage-current characteristic of a deuterium lamp. The lamp 10 in FIG. 1 follows such a characteristic. Once the gas has ionized, the voltage required to sustain the lamp is significantly less than the triggering voltage. The lamp 10 exhibits a non-constant negative resistance characteristic line 20 along which current increases and voltage decreases upon triggering of the lamp 10. As can be seen from the graph of FIG. 2, initially a high voltage but low current pulse is required to ionize the gas and subsequently a low voltage high current constant supply is required to sustain the operation of the lamp. For the circuit shown in FIG. 1 which uses a single power source 15 for energizing the lamp 10, in order to satisfy the above requirements, a series DC load impedance is provided by way of a resistor 18 which exhibits a voltage-current characteristic represented by the load line 22 in FIG. 2. The impedance matches the lamp operating impedance to the output voltage of the power source 15. The point of intersection A of the impedance load line 22 and the lamp voltage-current characteristic line 20 represents an unstable operating point once gas ionization has started, and the point of intersection B represents a stable operating point at which the deuterium lamp 10 sustains its discharge at the rated voltage and current.
The above described power supply circuit has several drawbacks. It can be appreciated that the series impedance must be chosen precisely to match the lamp current and voltage at the operating point B. Any variation in the load impedance will cause a change in the current supplied to the lamp 10 thereby changing the spectral wavelength output of the lamp at the operating point B. This is undesirable for spectrophotometer operations which typically require lamp outputs of constant wavelength for consistent spectral analysis.
The transition from trigger point A to stable operating point B should be smooth, which is difficult to obtain in the prior art circuit. The load line 22 from point A to point B does not track the voltage-current characteristic 20 of the lamp. Thus one faces the risks of either the gas deionizing thereby turning off the lamp or overloading the lamp thereby shortening the life of the lamp.
Another drawback of the power supply circuit shown in FIG. 1 is its inefficiency. For example, a 27 watt deuterium lamp rated at a trigger voltage of 600 V and a stable operation at 90 V and 0.3 A current requires a series resistance of 1700 ohms to match the voltage-current characteristic of the lamp. Such a power supply is very inefficient, requiring a large 180 watt DC supply in order to be able to provide the 600 V trigger voltage and 0.3 A lamp sustaining current to drive the 27 watt lamp, an efficiency of 15%. The large power supply is bulky and increases the cost of the spectrophotometer. A substantial amount of heat is dissipated by the series resistor 18, accounting for most of the 85% loss in efficiency. This heat could affect surrounding circuit components.
Another power supply circuit used in the past is shown in FIG. 3. Prior to triggering the lamp 30, the switch 34 is set at the position shown by the dotted line 36 so that the high voltage DC source 31 charges a capacitor 32 to an energy level sufficient for triggering the lamp. Thereafter, the switch 34 disconnects the capacitor charging circuit and closes the triggering circuit. The capacitor discharges, triggering the operation of the lamp 30. Thereafter a constant current DC power supply 38 continues to provide power through a series load resistor 40 for sustaining operation of the lamp 30 at the rated current. The sustaining voltage requirement of the power source 38 is much lower in this circuit than that shown in FIG. 1. The series impedance value can be reduced in this circuit to between 100 and 150 ohms for a typical 0.3 A, 90 V lamp. Efficiency is increased significantly over the circuit of FIG. 1 to as high as 50%. However, since the efficiency is a function of the input voltage of supply 38, it may drop below 35% for high values of supply input voltage.
While the efficiency has increased for the power supply of FIG. 3, it requires, however, two separate power sources to perform the functions of triggering and sustaining the lamp operation. Like the circuit shown in FIG. 1, the load resistor 40 of this circuit has to be chosen carefully to match the operating point B along the voltage-current characteristic 20 of the lamp. It is difficult to control a smooth transition from trigger point A to operating B when switching from the trigger DC source 31 to the current supply 38. If the impedance value changes at the lamp's operating point B, e.g. caused by circuit component deterioration or drift due to aging or temperature effect, current will change thereby causing changes in the spectral frequency output. If the current delivered to the lamp exceeds its rating, the life of the lamp will be shortened. Moreover, since the cathode temperature affects the lamp current, it would be desirable to provide a means of regulating the current power supply to provide a constant current to the deuterium lamp at its stable operating point to produce a constant spectral output.